Of the estimated 800,000 to 900,000 Americans who are currently infected with HIV, it is believed that as many as 275,000 remain unaware of their infection and are therefore without access to life-prolonging care or counseling that could prevent the further spread of the virus. This sobering fact suggests that it is time to reconsider the public health approach to HIV counseling, testing, and referral (HIV CTR) in the United States and to quantify the consequences of failing to provide adequate screening, treatment, and prevention. In the last seven years, our research group has published eighteen peer-reviewed policy evaluations using a computer simulation model of the clinical management and economics of HIV. Our findings have had a direct impact on HIV patient care decisions in the United States. We now propose to expand our model in order to address questions of design and priority setting in HIV CTR. We have two specific aims: (1) we will link our existing disease simulation to a new "screening/intake" model that can portray a broad spectrum of at-risk target populations, clinical settings, and testing/counseling protocols. We will codify the various steps that lead from detection of HIV, through treatment and counseling, to changes in viral infectivity, risky behavior, and disease transmission. We will assemble the most up-to-date clinical, epidemiological, economic, and behavioral data within a single, integrated framework. This will create the first comprehensive, model-based tool for the evaluation of HIV counseling, testing, and referral. (2) We will conduct and disseminate three policy evaluations assessing the impact of expanded HIV CTR services. a) We will evaluate the clinical and economic effects of conformity with the CDC's recently promulgated draft Revised Guidelines for HIV Counseling, Testing, and Referral. b) We will conduct an extensive assessment of alternative CTR intervention designs and provide practical guidance to decision makers wishing to tailor program attributes to population-, and setting-specific needs. c) We will perform the first formal exploration of the transmission benefits and risks of expanded HIV CTR in the HAART era. The investigators have a proven record of producing and disseminating findings that have helped to set priorities in HIV patient care, clinical trial design, and guideline development. Our aim, in the present proposal, is to exert an equally profound impact on priority setting in HIV testing and prevention.